In general, this invention relates to chemical sensing, and in particular, to referencing, rapid sampling and methods of reducing or eliminating drift in artificial olfactometry.
An electronic nose is an array of chemical sensors coupled with computerized multivariate statistical processing tools. These sensors respond to a wide variety of analytes giving rise to a unique signature or pattern for a given analyte. The pattern is interpreted using pattern recognition algorithmns to identify or quantify the analyte of interest.
In general, the chemical sensors are based on physical or chemical absorption, chemical desorption or optical properties that take place on the sensors. Suitable sensor types include metal oxide semiconductors, metal oxide semiconducting field effect transistors, conducting organic polymers, quartz microbalance, surface acoustic wave devices and conducting and nonconducting regions sensors.
For the analysis of organic solvent vapors, certain devices, such as surface acoustic wave devices, respond to the extent of vapor sorption. This sorption is typically rapid, reversible and is proportional to vapor concentration. However, various drawbacks exist. For example, certain sensors are susceptible to humidity, have low confidence limits, are susceptible to drift and are unstable. In certain instances, instability can be corrected using background subtraction techniques. Humidity in the vapor can be eliminated by using a preconcentrator with water vapor absorbents. Confidence limits can be enhanced by using a limit of recognition that is defined as the concentration below which a vapor can no longer be reliably recognized from its response pattern (see, Zellers et al., Analytical Chemistry, 70, 4191-4201 (1998)).
Drift is one of the most serious drawbacks of sensor technology. Drift is defined as the temporal shift of sensor response under constant or static conditions. The reason for certain types of drift is not well understood, but it is believed to result from unknown dynamic processes. Temperature or pressure fluctuations, or changes in the sensing environment can also cause drift. When the reasons for drift are known, it is sometimes possible to develop mathematical models that can compensate for its effects (see, Semin et al., Meas. Techn. 38, 30-32 (1995)). Work has been done on ways to improve the stability of sensors; however, it is not yet possible to fabricate sensors with no drift at all.
One possible solution to the effects of drift is to use a reference gas (see, Fryder et al. Transducers ""95 and Eurosensors IX., Stockholm Sweden, pp. 683-686 (1995)). This technique is difficult or impractical in some situations, such as a handheld sensing device. Another technique is the use of a theory of hidden variable dynamics for the rejection of common mode drift. Moreover, the hidden variable approach can be couple with adaptive estimation methods to compensate for drift (see, Holmberg et al., Sensors and Actuators B 42, 285-294 (1997).
In view of the inherent instability of certain sensor arrays, there remains a need to have effective referencing and calibration in spite of the presence of drift. Devices and methods are needed which effectively produce reliable vapor measurements in the presence of drift. The present invention fulfills these and other needs.
Temporal shift of sensor response under constant conditions is one of the most serious drawbacks of sensor technology. Devices and methods are needed which are effective to produce reliable vapor measurements in the presence of temporal shift.
As such, in certain aspects, the present invention provides a method for reducing drift in an artificial olfaction device having an array of sensors, the method comprising: contacting the array of sensor with an analyte at a first temperature to produce a first response; contacting the array of sensor with the analyte at a second temperature to produce a second response; and subtracting the first response from the second response thereby reducing drift in the sensor array. In certain instances, the artificial olfaction device is a handheld device.
In another embodiment, the present invention provides a sensor module configured for external mounting on a sensing apparatus for detecting an analyte in a fluid, the sensor module comprising: a casing sized and configured to be received in a receptacle of the sensing apparatus; at least two sensor to provide a distinct response when exposed to one or more analytes located within the sample chamber; and an electrical connector configured to be releasably engageable with a mating electrical connector of the sensing apparatus when the sensor module is received in the receptacle, the electrical connector transmitting the characteristic signals from the at least two sensor to the sensing apparatus.
In yet another embodiment, the present invention provides a sensing device for detecting an analyte, comprising: a housing; a first sensor element incorporating a first array of sensors and a second sensor element incorporating a second array of sensors wherein both sensor elements are mounted externally on the housing. In certain embodiments, the first sensing element is designed to sense a vapor, and is referred to as the sensing element. The second sensor element is designed as a reference for the device and is referred to as the referencing element. In certain aspects, a physical barrier exists between the reference sensor element and the analyte to be identified. Preferably, the reference element is pasivated with a material to prevent the analyte from contacting the surface of the reference element.
In still yet another embodiment, the present invention provides a sensing device for detecting an analyte, comprising: a housing; a sensor module mounted externally on the housing and incorporating an array of sensors, each of the sensors providing a different response in the presence of the analyte; a monitoring device mounted on the housing and configured to monitor the responses of the array of sensors incorporated in the sensor module, and further configured to produce a corresponding plurality of sensor signals; and an analyzer mounted on the housing and configured to analyze the plurality of sensor signals to identify the analyte.
In still other embodiments, the present invention relates to mapping an x-y surface for detection of an analyte, the method comprising: moving in tandem at least two sensor arrays separated by a distance xe2x80x9cdxe2x80x9d across an x-y surface to produce a plurality of responses; analyzing the responses and thereby mapping the x-y surface for detection of an analyte. In certain preferred embodiments, the tandem sensor system resides on a x-y translational stage.
In yet another embodiment, the present invention provides a sensor module, such as in a handheld device, comprising at least two pneumatic vapor paths and at least two sensors arrays. The dual pneumatic train allows rapid sensing as it increases the duty cycle frequency.
These and other aspects of the present invention will become more apparent when read with the detailed description and figures that follow.